The invention relates to a flexible abrasive body including a flexible abrasive medium having a pliable backing which exhibits one layer made from a pliable substrate on one side of which there is a full-coverage first metal coating in which the abrasive material is at least partly embedded.
Flexible abrasive bodies include, for example, abrasives on underlays, like endless abrasive belts or abrasive sheets fitted to a pliable support. Crucial to the durability of such flexible abrasive bodies is that their pliable support resists the tensile, compressive and shear forces without impairment during the grinding process and that when in use the vital abrasive grain does not become detached too quickly from the composite material. In addition, the thermal stability of the flexible abrasive body with respect to fixing of the grain and load-carrying capacity of the support must be sufficient to withstand the high temperatures that occur during grinding especially in dry-grinding operations. The super cutting materials diamond and CBN (cubic boron nitride), distinguished by their high thermal conductance and extremely high hardness, require the grain embedment to exhibit a particularly high heat resistance. Owing to the good cutting ability of those abrasive grains, even when used to grind the very hardest of materials, it is particularly necessary that the heat generated at the grain by the grinding process be conducted to the grain bonding agent layer and into the pliable support in order to avoid excessive, damaging temperatures in the workpiece and severe heat-induced grain damage. To accomplish this it is known to electrostatically embed the abrasive grain in thermally stable, durable metal, primarily nickel; cf. DE 1 059 794, EP 276 946, EP 0 263 785, EP 0 280 653, EP 0 013 486, DE 39 15 810, which are described in more detail in the following.
The electrostatic abrasive coating has only one layer of abrasive. The layer of metal or nickel building up from the support interlocks with the grains which are being progressively spread in parallel fashion, whereby tee depth of embedment of the required cutting grain can be controlled exactly via the duration of the electrodeposition process. Owing to the single-coat nature of the abrasive layer, electrostatically bonded abrasive grain cannot be dressed; at best it is possible to compensate for differences in the height of the grain peaks by way of spot-grinding. This inability to rework electrostatically bonded abrasive bodies leads to a typical feature, namely that the compactness of the abrasive layer is at best only as good as the compactness permitted by the underlying support. For the relevant grain sizes (e.g. 20-600 xcexcm with corresponding electrostatic embedment depths, e.g. 50-80%), a full-coverage, electrostatic metal bonding agent layer already exhibits a thickness which gives the planar structure the physical character of sheet metal. The thinner such layers, the higher is their flexibility or their fatigue strength under bending stress reversal because the relative difference between compressing and stretching the two sides of the planar structure decreases and the fatigue rupture under alternating loads is delayed. However such thin metal bonding agent layers just a few microns thick only have the ability to adequately anchor grain sizes of this order of magnitude. The strength and flexibility of electrostatic coatings may vary greatly, from stiff to brittle, almost to the suppleness of stress-free annealed rolled sheets, depending on bath composition, temperature, current densities and rate of deposition. Typically though, metal layers as thin as foil always exhibit a high sensitivity to impacts and buckling loads as well as low resistance to tear-propagation loads, which can be attributed to the low elastic deformability of the metal. Such irreversible, plastic deformations in a full-coverage, electrostatic grain bonding agent layer rule out its use as a highly durable, flexible abrasive body.
From DE 1 059 794 it is known to form a pliable support in the form of a metal layer on a flexible, endless steel belt which circulates in an electroplating solution and is wired as a cathode; spread over The surface of said belt is an abrasive grain bonded by means of an electrodeposited metal layer. This method creates a serviceable abrasive belt in the form of a metal foil with partly embedded abrasive grain when this abrasive coating is removed from the steel belt. The level of strength and the aforementioned problem of thin metal foils restrict the use of such abrasive belts to the very lightest of grinding operations or, owing to the limited flexibility, only the very thinnest electrostatic grain bonding agent layers and the very finest abrasive grits can be processed in this way to form flexible abrasive bodies. This abrasive coating can be laminated onto an abrasive support as a metallic coating. Although laminating the full-coverage electrostatic abrasive coating reduces the buckling susceptibility and raises the tearing strength, laminated flexible abrasive belts in continuous use generally suffer again and again from the problem that the extension ratios and the extension behaviour of the bonded layers differ. For instance, the use of laminated belts on grinding machines in which reversal and straight-running take place in dynamic alternation, the layer facing outwards is constantly subjected to tension and loading whereas at the same time the inner layer is constantly subjected to compression and relief. These differing longitudinal ratios must be compensated for elastically by the laminating adhesive. Furthermore, the extension behaviour of the different materials used for inner and outer layers differs considerably, like in the case of the electrostatic metal grain layer laminated onto an abrasive support discussed here. Durable laminated flexible abrasive bodies are only obtainable when the reversing radii are as large as possible and the laminated product is not too thick, because otherwise inner and outer belt lengths differ too greatly and adhesives that exhibit a mediating extension ability need to be employed. As a rule, the adhesive represents the weakest link in the planar composite system, meaning that just localized damage to the electrostatic abrasive coating leads to peeling and delamination of the whole coherent abrasive coating. To solve the problem of the lack of flexibility and the sensitivity of full-coverage, thin metal layers or metallic grain bonding agent layers in flexible abrasive media, various proposals have been made whose common feature is to refrain from using a full-coverage electrostatic abrasive coating on the surface of the flexible abrasive medium and instead to form the abrasive coating only at discrete, separate positions, i.e. isolated islands of abrasive coating arranged in regular patterns on a flexible substrate, e.g. cloth, whereby these isolated abrasive coatings are positioned on the surface offset with respect to each other in such a way that, in the direction of use, they overlap or abut. The interruption of the electrostatic coating, which becomes more and more rigid as the grain size and thickness of the layer increases, leads to the desired flexibility being essentially dependent on the underlying substrate because this has the chance to bend between the regularly arranged, discrete zones of abrasive coating.
For instance, a flexible abrasive medium is known from EP 0 280 657 in which a thin metal foil, copper in particular, is laminated onto a flexible, electrically non-conductive substrate so that a backing in the form of a planar composite material is obtained, one side of which is electrically conductive over its entire surface and the other side is electrically isolated. First, an electrically non-conductive mask with discrete openings is placed on the electrically conductive side and then metal, preferably nickel, is electrodeposited onto this together with the abrasive grain. During the electrodeposition process, the formation of the abrasive layer is restricted to the discrete openings in the mask so that islands of metal (nickel) abrasive coating and embedded grain are formed and not a complete covering. Thereafter, the mask separating the discrete abrasive zones is removed and the underlying metal foil still present etched away. Finally, the intermediate spaces are filled with resin and, if necessary, with silicon carbide powder. Instead of using a laminated metal foil, a metal layer can be attached directly to the substrate by means of a metallization process (electrochemical deposition without external currents, vapour-deposition or sputtering) and then processed further, as described, to form a flexible abrasive medium. The disadvantage of this is that in contrast to a smooch, laminated metal foil, the possible unevenness of the underlying substrate is not compensated for by the metallization process. This is not significant in the case of a flat, smooth substrate, e.g. foil or similar, but is significant for a substrate such as cloth, characterized by the loops of thread and undulations in the weave. An island-like coating of constant height cannot be built up on such an undulating, metal-coated cloth substrate, meaning that the clear projection of the embedded grain above the flexible abrasive body is not consistent either. The most severe disadvantage of this arrangement is that the island-type coating, which represents a heap of substrate, laminate adhesive if necessary, metal layer and metal bonding agent with grain, causes an overturning moment during grinding operations due to shear on the islands, and this can lead to them being easily torn away from the support. Filling the spaces between the islands with resin or resin and silicon carbide tiller is an attempt to reinforce this weak spot. The etched-off formally continuous metal or copper layer is interrupted to improve the flexibility, meaning that the thermally isolated island-like abrasive coating only permits a poor, interrupted transfer of heat into the flexible support.
A flexible abrasive body is known from EP 0 2163 785 in which a cloth substrate is used which is made electrically conductive by metal vapour-deposition or by interweaving metallic thread, or is formed by a metallized resin lattice. A mask containing discrete openings and made from polymer, electrically isolating resin is applied to this cloth under the action of heat and pressure. Metal, particularly nickel, is electrodeposited in the discrete openings in the presence of abrasive grain, thereby forming discrete abrasive coatings of deposited metal (nickel) and embedded grain. However the deposited metal adheres directly to the metallized cloth so the risk of the islands of abrasive grain becoming detached by shear forces during the grinding process is diminished. In this case the metallized fibres form thermal bridges between the individual abrasive coatings, although the conductance is low owing to the small cross-section of the fibres. The disadvantage of this version is that the undulations in the cloth prevent a consistent height being achieved for the island-type abrasive coatings. From this publication it is also known to mask an electrically conductive or non-conductive substrate in the form of a cloth in the aforementioned manner so that, again, openings are created for the electrostatic fixing of the grain. This masked cloth is fastened to a electrically conductive drum such that it cannot slip. The smooth drum wired as a cathode has the effect of causing the metal or nickel deposition to take place from its surface through the discrete openings of the cloth and the grain is attached only after the metal or nickel layer has interpenetrated the cloth. Upon completion of the electrostatic coating the flexible abrasive body is removed from the drum and can be laminated onto a stronger support.
This method can also be carried out continuously according to EP 0 276 946 when, instead of the rotating drum, an endless steel belt temporarily placed in non-slip, contact with the masked cloth, is passed through the electrolytic bath. The steel belt used as conveyor belt and cathode within the bath is separated from the flexible abrasive body outside the bath upon completion of the electrodeposition process and as a circulating belt picks up new cloth again at the entrance to the bath.
Advantageous with these flexible abrasive bodies according to EP 0 276 946 and the second version of EP 0 263 785 is that the metal-based island-like abrasive coating interlocks with the cloth from top to bottom and hence reduces the risk of the island-like abrasive coating being torn off by the overturning moment brought about by the grinding process. However like with all other arrangements of island-type, discrete abrasive coatings, the weak spot here is again the zones between the islands, the zones devoid of grain and metal or nickel. Here too the island-like abrasive coatings do not form thermal bridges with each other so the heat generated by the grinding process builds up in the island-like abrasive coatings. Another disadvantage is that only extremely thin, net-like, open, light cloths allow the metal (nickel) to interpenetrate evenly and interlock because the threads represent per se disruptions in the electrodeposition and thick electrostatic coatings generally cannot be produced without imperfections and also not consistently thick. The island-like, disc-like metal or nickel coatings attached by means of the smooth drum cathode or the smooth steel belt cathode lose their shape more and more towards the build-up side the thicker the layers become or at that moment when the cloth has been completely interlocked and enclosed. This means that after the cloth has been penetrated the metal- or nickel-coat discs present are not flat and not consistently thick and so cannot be used as a base for bonding abrasive grain electrostatically. The flexible abrasive body obtained in this way exhibits a low level of strength owing to the limited cloth thickness and cloth make-up and must be laminated onto a thicker support to provide strength. This increases further the thickness tolerance of the flexible abrasive body. In addition, laminating certainly improves the compressibility of the planar composite material in comparison to the individual components. This coating underneath the practically incompressible, disc-like metallic abrasive coatings places them on a more or less elastic base which rules out dimensionally accurate grinding.
A similar flexible abrasive body is known from E 0 013 486. An electrically non-conductive mask, with discrete openings for electrodeposition, is fitted to an electrically conductive drum. An electrically non-conductive cloth spanned over the drum wired as the cathode is only interpenetrated by electrodeposited metal (nickel or copper) at the discrete positions predetermined by the mask. After penetrating the cloth, grain is spread on and embedded in the growing layer of metal. Finally, the flexible abrasive body is removed from the drum and processed further. This flexible abrasive body is essentially only distinguished from the abrasive body according to EP 276 946 by the fact that the desired disc-shaped metal deposition is only aligned by the mask on the drum and no longer by the interpenetration of the cloth. Therefore, this abrasive body is only suitable for particularly fine, net-like cloths as flexible supports, e.g. when grinding lenses. In a modified version of this method, grain of equal height is achieved on the flexible abrasive body in an electrostatic but not single-coat grain layer. To do this, abrasive grain is first embedded electrostatically in the mask openings on the masked drum. When sufficient grain has been embedded, an electrically non-conductive cloth is laid on top and the metal deposition process started. After the cloth has been penetrated and the metal deposited to a certain thickness, the process is halted and the flexible abrasive body removed from the drum. The advantage of this arrangement is that a homogeneous rain height is achieved, but the grain is practically completely embedded and not very sharp for an electrostatically bonded grain, and hence only suitable for fine work. On the side of the flexible abrasive body away from the grain, the inconsistency of the disc-like abrasive coatings is again attributable to the build-up through the electrical disruption of the cloth. This means that adequate compactness of the flexible abrasive body cannot be accomplished.
A flexible abrasive body is known from DE 39 15 810 which has a pliable support of electrically conductive material (metal foil). Bonded or unbonded reinforcing threads are woven into the conductive material by means of overlapping seams. Further, the seams also connect a mat of non-conductive material on the other side of the metal foil with the metal foil. The upper side is isolated by a covering in discrete zones in such a way that between the reinforcing threads metal foil remains exposed onto which the metal is electrodeposited, forming protruding islands. Afterwards, a stabilizing coating of synthetic resin is applied to both sides of the support. The coating covers the mat, fills the spaces between the islands and also covers the islands. The support is subsequently ground away on the island side so that the metal islands are exposed. Metal and abrasive grain are then electrodeposited onto the islands. The disadvantage is the large amount of electrodeposited metal required because the reinforcing threads and connecting threads must be covered before the abrasive grain can be embedded electrostatically. The underlying metal foil does not remain rigid permanently. Alternatively, the first electrodeposition can cover the whole surface, whereby the reenforcing threads represent electrical disruptions; but the support then has a very stiff, rather inflexible sandwich construction.
The task of the present invention is to specify an abrasive body of the aforementioned type having high thermal conductance, excellent flexibility, high dimensional stability and compactness, as well as method for its production.
This task is solved by the invention comprising a flexible abrasive body, comprising:
a) a pliable backing including one layer made from a pliable substrate on one side of which there is a full-coverage first metal coating in which the abrasive material is at least partly embedded; and
b) the backing including a substrate and first metal coating including a constant thickness, and the first metal coating including a constant thickness, and the first metal coating including a flat, smooth surface substantially free from disruptions, and a minimized coating thickness.
One or more of the following inventive features may likewise be included in embodiments according to the invention:
a second metal coating is provided with fractures;
the first metal coating is provided with fractures;
a second metal coating anchoring the abrasive material is arranged across the entire surface or at discrete points on the first metal coating;
the second metal coating is applied by way of electrodeposition on the first metal coating;
the substrate on the side opposite to the first metal coating has a coating of non-conductive material with a flat, smooth surface and minimized coating thickness;
the material of the coating is a resin;
reactive, cross-linkable precursors of thermosetting plastics, exhibiting a moldable, hardenable B-state, are used as hardenable resins;
the resin is phenolic resin;
the coating is provided with compression buckling points;
the substrate is textile structure;
the substrate is a cloth, braid, knitted fabric or non-woven fabric;
the substrate consists of thermally stable, organic, inorganic, metallized fibers or metallic fibers or a mixture thereof;
the first metal coating is made from copper;
the coatings are interlocked with the threads of the substrate;
the second metal coating is made from nickel;
the abrasive material is diamond or cubic boron nitride;
the thickness of the first metal coating and the thickness of the non-conductive material coating is 3-25 xcexcm at each of the highest elevations of the substrate (cloth, braid, knitted fabric, non-woven fabric);
the breaking points and/or buckling points ran essentially transverse to the envisaged grinding direction;
full-coverage coating of one side of a pliable substrate with an excess of a first metal to form a first metal coating;
removal and levelling of the metal down to a predetermined thickness of the support formed by the substrate and the first metal coating;
coating of the first metal coating with a second metal to form a second metal coating with simultaneous embedment of the abrasive material;
the first metal coating is removed and levelled to such an extent that the highest elevations of the substrate still remained covered with a thin layer of metal
the support on the side opposite to the first metal coating is provided with a coating made from a non-conductive material;
the support on the side opposite to the first metal coating is provided with a smooth, flat coating made from a non-conductive material;
the coating made from the non-conductive material is removed and levelled to such an extent that the highest elevations of the substrate still remain covered with a thin layer of material;
the thickness of the first metal coating and the coating of non-conductive material is in each case 3-25 xcexcm;
the non-conductive material is a hardenable resin, in particular phenolic resin;
the first metal coating and/or the abrasive material is/are fractured;
fracturing of the metal coatings is carried out before, during and after applying the second metal coating;
the coating made from non-conductive material is provided with buckling lines;
both metal coatings are brittle;
foreign particles are embedded in the metal coatings;
the second metal coating is electrodeposited on the first metal coating;
the first metal coating is applied from the solid, liquid, gaseous or dissolved state of aggregation;
a bonding agent is used to improve the adhesion between first metal coating and substrate;
the levelling is carried out by rolling, plating, pressing, forging or shot-blasting; and
the removal of the metal of the first metal coating and the material of the non-conductive coating is carried out by sand-blasting, milling, grinding, chemical or electro-etching, EDM or by cutting methods including one of laser, electron bear, and water jet.
The invention proposes providing a substrate, e.g. a textile structure such as cloth, knitted fabric, non-woven fabric, etc., with hard coating masses having smooth, flat surfaces and these to be on one or both sides such that on one side there is in electrically conductive material, preferably metal, e.g. copper, and in addition, if necessary, on the other an electrically non-conductive material, preferably a hardenable resin, e.g. phenolic resin. The substrate coated in this, way forms a support for abrasive grain and is dressed to a constant thickness such that the high spots of the support, at least on the metal-coated side, are still covered with a very thin layer of metal. The constrictions in the hard coating masses caused by the subsequent working (dressing) lend the support the necessary flexibility; on the other hand, a high compression resistance perpendicular to the support is preserved. Such ant arrangement is particularly advantageous when the substrate is a textile structure which exhibits undulations caused by the loop of thread, i.e. the thread crossings. The coatings here are fully interlocked with the threads. The highest thread elevations still remain, at least on the metal side, covered with a very thin layer of metal. i.e. approx. 3-25 xcexcm, while the main amount of electrically conductive material (metal) and the electrically non-conductive material is localized between the thread crossings. The support with constant thickness and smooth metallic surface produced in this way forms an ideal, homogeneous support for a full-coverage, electrostatic coating with a metallic embedment material, preferably nickel, and with abrasive grain, thus rendering possible the production of a flexible abrasive body characterized by a uniform grain height and grain embedment.
Flexibility is further increased by fracturing the metal coatings, such as by providing a second metal coating with fractures; providing the first metal coating with fractures, and providing the coating with buckling points.
The invention shall be explained in more detail by means of the accompanying drawing which shows in schematic form the structure of the flexible abrasive body according to the invention by means of its step-by-step production.